1. Field of Art
This invention relates to apparatus used in connection with the drilling of subterranean boreholes, which in the oil and gas industry are commonly called “wells.” With further specificity, this invention relates to a type of remotely operable safety valve disposed in the drill string below a top drive unit, the valve having more than one valve element therein.
2. Description of the Related Art
The use of so-called “top drive units” on rotary drilling rigs, in the drilling of oil and gas wells, has become quite common. Rather than utilizing the rig's rotary along with a kelly bushing and kelly to rotate the drill string, the top drive rotates the drill string by a large electric motor mounted in an assembly which is raised and lowered by the rig's traveling block. FIGS. 1 and 2 show the general layout. Typically, the traveling block runs on a pair of guide rails to hold it oriented over the borehole. Drilling fluid is pumped down through the main shaft of the top drive.
The typical sequence of equipment from the bottom of the main shaft of the top drive to the top of the drill string includes (in descending order): the threaded end of the main shaft; the remotely operable top drive safety valve; the manually operable top drive safety valve; a saver sub; and the uppermost “box” connection of the drill string. FIG. 3 shows this sequence of equipment. In the normal course of drilling, both the remotely operable and manually operable top drive safety valves must be open to permit drilling fluid to be pumped down the drill string. Both top drive safety valves have valve elements therein to control fluid flow, which are typically (but not necessarily) “ball valves,” which, as is well known in the art, employ a generally spherical “ball” having a bore through it and attached to a valve stem. Rotating the stem (usually by means of an external arm connected thereto), and consequently the ball, so that the bore of the ball is aligned with the bore of the valve body opens the valve element and permits fluid flow therethrough. Rotating the ball typically ¼ rotation from the open position presents a solid surface of the ball to the valve bore, thereby shutting off fluid flow through the valve. Other types of valve elements may be used (butterfly, gate, etc.)
The top drive safety valves are very important devices to control fluid flow and pressure. Regulations require that at least one of the top drive safety valves be remotely operable, by some powered means. In this description, by way of example, the uppermost top drive safety valve is shown as the remotely operable top drive safety valve (however, it is understood that the remotely operable top drive safety valve could be positioned above or below the manually operated safety valve). Remotely operable top drive safety valves usually employ a longitudinally slidable actuator sleeve which operatively engages the external arm attached to the stem of the valve element, to open and close it. The actuator sleeve is moved up or down by a powered (typically pneumatic or hydraulic) operating arm. Known prior art remotely operable top drive safety valves comprise only a single valve element therein.
Regulations further require that the top drive safety valves be periodically function and pressure tested. Should the remotely operable top drive safety valve fail the pressure test (that is, even when the valve element is closed, it is not pressure and flow tight), the valve must be repaired, usually done by removing the valve from the top drive assembly and replacing it with a standby valve, while the malfunctioning safety valve is sent to a repair facility. Significant time is needed to remove the top drive safety valve from the drill string. While the top drive safety valve is being removed, and another one installed, drilling operations are at a standstill. With many offshore drilling operations having overall daily costs of $100,000 or more, it is readily appreciated that downtime associated with replacement of top drive safety valves is very costly.
Therefore, a need exists for a remotely operable top drive safety valve, especially for use in conjunction with top drive drilling units, comprising multiple valve elements, so that when one valve element fails to pressure test, the second (or third, etc.) valve element can be put into service without removing the top drive safety valve from the drilling assembly. The dual valve elements will permit at least twice the service time as with a top drive safety valve comprising only a single valve element.